3 marksShort AnswerSection B

(a) Draw two different possible structures of a saturated hydrocarbon having four carbon atoms in its molecule. What are these two structures of the hydrocarbon having same molecular formula called? Write the molecular formula and the common name of this compound. Also write the molecular formula of its alkyne. OR (b) (i) Write the molecular formula of benzene and draw its structure. (ii) Write the number of single and double covalent bonds present in a molecule of benzene. (iii) Which compounds are called alkynes?

Q: (a) Draw two different possible structures of a saturated hydrocarbon having four carbon atoms in its molecule. What are these two structures of the hydrocarbon having same molecular formula called? Write the molecular formula and the common name of this compound. Also write the molecular formula of its alkyne. OR (b) (i) Write the molecular formula of benzene and draw its structure. (ii) Write the number of single and double covalent bonds present in a molecule of benzene. (iii) Which compounds are called alkynes?

### **Option (a)** 1. **Two structures of a saturated hydrocarbon with four carbon atoms (Butane):** - **Straight chain (n-butane):** $$\text{H}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\text{H}$$ - **Branched chain (isobutane):** $$\text{H}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{CH}_3}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\text{H}$$ 2. **Name of these structures:** These structures, which have the same molecular formula but different structural arrangements, are called **structural isomers**. 3. **Molecular formula and common name:** - **Molecular formula:** $\text{C}_4\text{H}_{10}$ - **Common name:** **Butane** 4. **Molecular formula of its alkyne:** The alkyne with four carbon atoms (butyne) has the molecular formula **$\text{C}_4\text{H}_6$** (following the general formula $\text{C}_n\text{H}_{2n-2}$). --- ### **Option (b)** (i) **Molecular formula of benzene:** $\text{C}_6\text{H}_6$ **Structure of benzene:** ```molecule {"smiles": "c1ccccc1", "caption": "Benzene (C6H6)"} ``` (ii) **Number of covalent bonds in benzene:** - **Single covalent bonds:** $9$ (comprising $3$ carbon-carbon single bonds and $6$ carbon-hydrogen single bonds). - **Double covalent bonds:** $3$ (carbon-carbon double bonds). (iii) **Alkynes:** Unsaturated hydrocarbons containing one or more triple bonds between carbon atoms are called alkynes. Their general formula is $\text{C}_n\text{H}_{2n-2}$.

Carbon and its Compounds

Isomerism, Benzene, Alkynes

Official Answer

Option (a)

Two structures of a saturated hydrocarbon with four carbon atoms (Butane):

Straight chain (n-butane):

$\text{H}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\text{H}$

Branched chain (isobutane):

$\text{H}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{CH}_3}{|}}{\underset{\text{H}}{\text{C}}}-\overset{\underset{\text{H}}{|}}{\underset{\text{H}}{\text{C}}}-\text{H}$

Name of these structures: These structures, which have the same molecular formula but different structural arrangements, are called structural isomers.

Molecular formula and common name:

Molecular formula: $\text{C}_4\text{H}_{10}$
Common name: Butane

Molecular formula of its alkyne: The alkyne with four carbon atoms (butyne) has the molecular formula $\text{C}_4\text{H}_6$ (following the general formula $\text{C}_n\text{H}_{2n-2}$ ).

Option (b)

(i) Molecular formula of benzene: $\text{C}_6\text{H}_6$

Structure of benzene:

Benzene (C6H6)

(ii) Number of covalent bonds in benzene:

Single covalent bonds: $9$ (comprising $3$ carbon-carbon single bonds and $6$ carbon-hydrogen single bonds).
Double covalent bonds: $3$ (carbon-carbon double bonds).

(iii) Alkynes: Unsaturated hydrocarbons containing one or more triple bonds between carbon atoms are called alkynes. Their general formula is $\text{C}_n\text{H}_{2n-2}$ .

structural isomersC4H10ButaneC4H6C6H69 single bonds3 double bondstriple bondalkynes

Marking Scheme

1Option (a): Drawing the straight and branched structures of butane correctly (1 mark)
2Option (a): Identifying them as structural isomers and writing the formula C4H10 / name Butane (1 mark)
3Option (a): Writing the molecular formula of its alkyne as C4H6 (1 mark)
4Option (b)(i): Writing the molecular formula of benzene (C6H6) and drawing its ring structure (1 mark)
5Option (b)(ii): Correctly stating 9 single bonds and 3 double bonds in benzene (1 mark)
6Option (b)(iii): Defining alkynes as unsaturated hydrocarbons with triple bonds (1 mark)

Hint

For (a), draw a straight chain of 4 carbons and a branched chain of 3 carbons with 1 methyl group. For (b), remember that benzene is a ring of 6 carbons with alternating double bonds, and do not forget to count the C-H single bonds.

Quick Oral Answer

Butane has two structural isomers: n-butane and isobutane, both sharing the formula $\text{C}_4\text{H}_{10}$ . Benzene has the formula $\text{C}_6\text{H}_6$ and contains 9 single bonds and 3 double bonds in its cyclic structure. Alkynes are unsaturated hydrocarbons containing at least one carbon-carbon triple bond.

Analysis & Explanation

For Option (a), butane ( $\text{C}_4\text{H}_{10}$ ) has a four-carbon chain that can be arranged either linearly (n-butane) or with a branch (isobutane). These are structural isomers. The corresponding alkyne has two fewer hydrogen atoms than the alkene, giving the formula $\text{C}_4\text{H}_6$ .

For Option (b), benzene ( $\text{C}_6\text{H}_6$ ) is a cyclic hydrocarbon. Its structure consists of a ring of six carbon atoms with alternating single and double bonds. Each carbon is also bonded to one hydrogen atom. This results in $6 \text{ (C-H)} + 3 \text{ (C-C)} = 9$ single bonds and $3 \text{ (C=C)}$ double bonds.

Common Mistakes

1Forgetting to count the C-H single bonds in benzene, leading to an incorrect count of single bonds (e.g., writing 3 instead of 9).
2Writing the formula of the 4-carbon alkyne as $\text{C}_4\text{H}_8$ (which is an alkene) instead of $\text{C}_4\text{H}_6$ .

Interesting Facts

Benzene's unique ring structure with alternating double bonds was famously envisioned by chemist August Kekulé in a dream of a snake biting its own tail.

Isobutane is commonly used as an environmentally friendly propellant in aerosol sprays and as a refrigerant.

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Frequently Asked Questions

How many marks does this question carry in CBSE Class 10 Science 2022?

This question carries 3 marks in the CBSE Class 10 Science 2022 examination.

Which chapter does this question come from in Science?

This question is from the chapter "Carbon and its Compounds" in the CBSE Class 10 Science syllabus.

What topic does this question cover in Science?

This question covers the topic "Isomerism, Benzene, Alkynes" from CBSE Class 10 Science.

What type of question is this in the CBSE Class 10 Science 2022 paper?

This is a Short Answer question from Section B in the CBSE Class 10 Science 2022 paper.